Computer disk drives, e.g., hard disk drives, use a motor to turn at least one magnetic disk at high speeds. Typically, the magnetic disk is coupled to a rotor, which, in turn, is coupled to the motor for rotating the magnetic disk via the rotor. A stator is also provided to retain the rotor. A bearing serves as an interface between the stator and the rotor. During operation, lubricant is typically required and may be provided in the bearing. For example, the bearing may include a pair of magnetic fluid seals, several capillary holes or channels, and a lubricant flowing within the capillary channels that is held in place by a barrier film.
Disk drive systems also include at least one read/write transducer or “head” that serves to read and write data to the magnetic disk. The transducer may be supported by an air-bearing slider that has a top surface attached to an actuator assembly via a suspension and a bottom surface having an air-bearing design of a desired configuration to provide favorable flying height characteristics. As a disk begins to rotate, air enters the leading edge of the slider and flows in the direction of the trailing edge of the slider. The flow of air generates a positive pressure on the air-bearing surface of the slider to lift the slider above the recording surface. As the spindle motor reaches the operating velocity, the slider is maintained at a nominal flying height over the recording surface by a cushion of air. Then, as the spindle motor spins down, the flying height of the slider drops.
A problem common to many disk drive systems relates to the tendency of the system to accumulate static charge. Such charge may build up, uncontrollably discharge, and damage various components of the disk drive. For example, a voltage difference between the heads and disks can provide a potential energy discharge that can harm the heads and the disk media itself. Although the initial voltage differences between components of a disk drive system can be kept to near zero, static charging typically accumulates in a disk drive system during operation, thereby building up voltage differences between components of the disk drive system. Such voltage differences occur between moving parts such as between the head and the disk and/or between the rotor and the stator.
A number of sources may contribute to static charge accumulation. When surfaces rub against each other in a bearing, electron and ion exchange or charge separation may occur, thereby resulting in tribocharging. Even occasional contact of asperities on the heads and disks can generate tribocharge. Due to the close proximity of surfaces in a disk drive platter, even a small charge imbalance forms potential differences on the order of volts. In addition, charge accumulation may occur because of shearing of air molecules in the boundary layer adjacent to rotating surfaces. Charging also occurs due to shear flow of the motor bearing lubricant. Furthermore, lubricant used in the bearing may be dielectric in nature. Dielectric lubricants generally lack the capability to allow for dissipation of accumulated charge. Because shear forces may break chemical bonds in the lubricant, liberated electrons and ions may further contribute to charge accumulation.
One known approach to alleviate excess static charging is to provide a lubricant that has a sufficiently high electrical conductivity to dissipate charge accumulated in a disk drive charge. A number of patents describe additives for increasing the electrical conductivity of a lubricant. For example, U.S. Pat. Nos. 5,641,841, 5,744,431, and 5,886,854, each to Diaz et al., describe various conducting polyaniline derivatives that may be used to render a lubricant conductive. Additional patents that describe conductive lubricants include, U.S. Pat. No. 5,773,394 to Wan et al., U.S. Pat. No.5,940,246 to Khan et al., and U.S. Pat. No. 6,250,808 to Ichiyama. It is thought that highly conductive lubricants in the bearing would reduce any potential difference that may build up between a rotor and a stator.
Nevertheless, it has been found that lubricant conductivity represents only one of many factors that influences the static charging behavior of disk drive systems. In particular, it has been discovered that disk drives that employ highly conductive lubricants may nevertheless experience a surprisingly high peak discharge current because the lubricants themselves may accumulate excessive static charge. Accordingly, there is a need for a disk drive lubricant that is not prone to accumulating excessive static charge. Such lubricants typically have a low relative electrical permittivity and may be formulated, for example, by introducing a charge-control additive for reducing charge accumulation into an ordinary lubricating medium.